The flat panel displays (FPDs) used in personal computer monitors, televisions, and cellular phones are constructed from display elements such as liquid crystal or electroluminescent (EL) elements and a thin-film transistor array (TFT array) for electrically controlling the states of the display elements. As shown in FIG. 1, the TFT array substrate 16 is configured with a plurality of pixels 27 arranged in a matrix. Gate control lines 22 and data lines 20 are disposed horizontally and vertically and connected to the pixels 27. Each pixel is controlled by selecting the pixel to be controlled by a gate control line 22 and a data line 20, and the display luminance is set by the voltage applied to the data line 20.
Over the past few years, self-emitting elements like organic EL elements have gained attention as display elements. A self-emitting element has the property of emitting light, has a wide displayed color range, and is suited to smaller and lighter weight FPDs. Therefore, a TFT array for self-emitting elements requires a control circuit for controlling the drive current of the self-emitting element by a voltage applied to the data line 20.
FIG. 2 is an example of the structure of a pixel 27 in a typical TFT array 16 for EL elements formed from two p-channel polysilicon TFTs. This example shows an example circuit configuration using p-channel TFTs, but can similarly be applied to n-channel TFTs. The case of using polysilicon for the silicon layer of the TFT is cited, but an amorphous silicon layer can be used.
The gate of a pixel selection transistor 23 is connected to the gate control line 22 and the drain to the data line 20. The source of the pixel selection transistor 23 is connected to the gate of a drive transistor 24 and a first electrode of a hold capacitor 25. The source of the drive transistor 24 and a second electrode of the hold capacitor 25 is connected to a power supply line 21. The drains of the drive transistors 24 are connected to the EL elements 26 when the FPD is completed, but the EL elements 26 in the TFT array 16 state are in the open state because the elements are not sealed.
Next, the operation of a pixel 27 is explained. Since the gate control line 22 normally has the off voltage (normal) in the range of 5 to 10 V applied by the positive power supply voltage of the logic circuit in the FPD, the pixel selection transistor 23 of each pixel enters the off state. When a pixel is controlled, first, the on voltage, for example, −5 V, is applied to the gate control line 22 connected to the pixel 27 (selection pixel) to be controlled. This places the gap between the drain and the source of the pixel selection transistor 23 in the conducting state. The voltage V corresponding to the desired emitted light luminance is applied to the data line 20. Then the hold capacitor 25 is charged, and the voltage between the gate and source of the drive transistor 24 is held in the difference between the potential of the power supply line 21 and the potential V of the data line 20. Since the hold capacitor 25 is connected to the gate and source of the drive transistor 24, the EL element drive current corresponding to the voltage V flows between the drain and source of the drive transistor 24. However, in the TFT array state, the drive current does not flow because the EL element is not sealed and the drain is in the open state.
A TFT array 16 is formed on a glass substrate. FIG. 3(b) is a cross-sectional view of the glass substrate forming the TFT array, and (a) shows the corresponding circuit. In the layout relationship shown in (a), the power supply line 21 is divided into two lines, but both lines are electrically connected and are the same line.
The control circuit of the TFT array 16 is formed on the glass substrate 30 coated with a cover coating layer 31. First, undoped polysilicon layers 23p, 24p are formed at the positions opposite the gate layers 23g, 24g of the transistors 23, 24, and p-type semiconductor layers (polysilicon layer doped with boron) are formed at the positions of the drains and sources. The hold capacitor 25 uses the polysilicon layer 25p at the position opposite the first electrode 25g as the second electrode, and the insulating layer 32 and the depletion layer possible in the polysilicon layer as the dielectric layer, to form the so-called MOS capacitor.
Each layer is covered by a first insulating layer 32, and metal wiring layers 20m, 28, 29, 21m are disposed at the drains 23d, 24d and the sources 23s, 24s, respectively. The metal wiring layers 20m, 21m are connected to the data line 20 and the power supply line 21, respectively. The gate layers 23g, 24g of the transistors 23, 24 formed from structural materials and the second electrode 25g of the hold capacitor 25 formed from the same structural materials are formed with the top layer of the first insulating layer 32. Although not shown, the gate layer 24g of the drive transistor 24 and the source layer of the pixel selection transistor 23 are electrically connected. To construct the circuit shown in FIG. 2, the metal wiring layer 21m and the second electrode 25g must also be electrically connected. However, the metal wiring layer 21m and the second electrode 25g do not necessarily have to be electrically connected, and a different voltage is sometimes applied depending on the usage state. A second insulating layer 33 is formed to cover the gate layers 23g, 24g and the second electrode 25g. Furthermore, a protective layer 34 is formed as the top layer.
As is clear from FIG. 3, the hold capacitor 25 is formed from the first electrode 25g and the second electrode 25p, and p-type semiconductor layer 23s is disposed adjacent to the second electrode 25p and opposite the metal layer 25g. This structure has the same structure as gate layer 24g and the polysilicon layer 24p in drive transistor 24 and the p-type semiconductor layers 24s, 24d disposed adjacent thereto. Thus, since the drive transistor 24 and hold capacitor 25 on the TFT array can be formed in the same structure, they are often fabricated by a common process.
The gate capacitor of the drive transistor 24 and the hold capacitor 25 formed by the common process and having the same dielectric material (insulating layer 32) and thickness of the insulating layer have nearly equal electrical characteristics such as the capacitance per unit area and the dependence of the capacitance on the voltage.
In this application, the structural materials are the materials forming the transistors or the electrodes of the hold capacitors. For example, the structural material of the gate of the pixel selection transistor 23 is metal for forming the gate 23g. The structural materials of the drain and source are p-type semiconductors forming the drain 23d and the source 23s. The structural material of the gate of the pixel selection transistor 23 does not necessarily have to be metal, but can be a material like tungsten silicon or polysilicon. Similarly, the structural material of the first electrode of the hold capacitor 25 is a metal forming electrode 25g, and the structural material of the second electrode is the p-type semiconductor forming electrode 23s. The structural materials, physical dimensions such as the film thickness, and the manufacturing method for forming the structural materials on a substrate are appropriately selected to match the electrical specifications demanded for the transistors and hold capacitors.
Because the TFT array substrate 16 has a wide area, it is difficult to manufacture with uniform electrical characteristics of the functional components (transistors and hold capacitors) on the substrate over the entire surface. Therefore, the problem is the resulting fluctuations in the drive current flowing between the drain and source of the drive transistor 24 in each pixel produce fluctuations in the luminance of the emitted light. If the fluctuations are small, this does not present a problem in practice, but fluctuations above a designated level are unsuited to products. Therefore, a decision about the quality of the manufactured TFT array is required.
The decision on the quality of the TFT array is desired before sealing the self-emitting material because self-emitting elements such as organic EL materials are usually expensive. In the state before sealing the EL elements 26, the problem is the drive current cannot be directly measured because the drain terminal of the drive transistor 24 is in the open state.